The invention herein disclosed and claimed is directed to the use of continuous wave (cw) frequency energy of high spectral purity in the fluid-coupled accurate measurement of the amplitude of low frequency vibration of an object or entity under investigation. In the description which follows, the invention will be described in conjunction with the accurate measurement of the amplitude of acoustically induced vibration of organs in a living fish, in which the living organism and the ultrasonic investigating mechanism are all coupled underwater, but it will be apparent to those skilled in the art that other and different applications may be made of the invention without departing from the principles disclosed herein.
In the field of measuring the amplitude of acoustically induced vibrations within a living organism, there is currently no technique available for use which does not at least require surgery. In fact, certain available techniques cannot be performed on living organisms. For the purpose of the description which follows, currently available techniques which require surgery or which cannot be performed on living organisms will be termed "invasive" techniques. In distinction, a technique which does not require surgery and which may be performed on a living organism will be termed "non-invasive". Invasive techniques or methods include the use of the Mossbauer effect, lasere vibrometry and holography and the use of accelerometers or microphones.
The invasive techniques involve surgery to expose the tissue to be investigated and, in the case of those employing the Mossbauer effect or of the use of accelerometers, the probe is required to be placed in contact with the tissue being investigated, which will invariably alter its vibrational characteristics. For the purpose of this description, a method which alters the vibrational characteristics of the organism and its tissue will be termed an "intrusive" technique, whereas a technique which does not alter the vibrational characteristics will be termed as "non-intrusive".